The present invention relates to a process for producing single crystal ingots having a reduced amount of crystal defects. More particularly, the present invention relates to a process for producing a silicon melt for growing single crystal silicon ingots wherein the silicon melt contains a very low amount of gases insoluble in silicon.
In the production of single silicon crystals grown by the conventional Czochralski method, polycrystalline silicon in the form of granular polysilicon, chunk polysilicon, or a mixture of chunk and granular polysilicon is first melted down within a quartz crucible and equilibrated at a temperature of about 1500xc2x0 C. Chunk polysilicon is a polycrystalline silicon mass which is generally irregular in shape, with sharp, jagged edges as a result of the fact that it is prepared by breaking rods of polycrystalline silicon into smaller pieces; chunk polysilicon typically ranges from about 2 centimeters to about 10 centimeters in length and from about 4 centimeters to about 6 centimeters in width. Granular polysilicon is a polycrystalline silicon mass that is generally smaller, more uniform and smoother than chunk polysilicon as a result of the fact that it is typically prepared by chemical vapor deposition of silicon onto a silicon granule in a fluidized bed reactor; granular polysilicon typically ranges from about 1-5 millimeters in diameter and generally has a packing density which is about 20% higher than chunk polysilicon.
As the polysilicon is heated and melted, an inert purge gas such as argon is continuously introduced over the crucible and silicon to remove unwanted contaminants from the melt area that are produced in and around the melt during the melting of the polysilicon. After the silicon has completely melted and reached a temperature of about 1500xc2x0 C., a seed crystal is dipped into the melt and subsequently extracted while the crucible is rotated to form a single crystal silicon ingot. During the early stages of the melting process when the polycrystalline charge is completely or partially in the solid state, the purge gas may become trapped in the polysilicon charge. The gas may be trapped between the individual polysilicon charge pieces themselves, or between the charge pieces and the sides or bottom of the crucible and eventually become insoluble bubbles in the melt which can be grown into the growing crystal. Although most of the insoluble bubbles, such as argon bubbles, present in the melt are released into the adjacent atmosphere during melting and temperature equilibration, some remain in the silicon melt and can be grown into the silicon crystal, thereby producing voids in the crystal.
While the problem of trapped gases occurs with all charge types including chunk silicon, polycrystalline silicon, and mixtures thereof, the problem is particularly acute with charges formed from only granulated polycrystalline silicon; the granular polysilicon with its high packing density tends to insulate the bottom and side walls of the crucible making it more difficult for insoluble gases such as argon to escape during the melting process. The purge gas, which has conventionally been argon because of its low price and non-reactive nature, is highly insoluble in silicon. Because argon is highly insoluble in silicon, trapped argon gas in the melt forms small bubbles in the liquid silicon during melting. Many of the insoluble gas bubbles contained in the liquid melt rise to the surface or are carried to the surface by convection and are released into the crystal growth gas ambient and thus have no detrimental effect on the growing ingot. A smaller number of the gas bubbles remain in the liquid melt throughout the pulling process and are grown into the crystal itself during growth. These bubbles, generally comprised of the insoluble argon purge gas, become trapped at the liquid-solid growth interface and cause large crystal voids on the crystal surface. Such defects are generally characterized and detected on sliced silicon wafers as large pits generally having a diameter of greater than about 1 micrometer. These pits are identified through laser scanning of polished wafers cut from the grown crystal. Such defects can effect 4% or more of wafers sliced from grown crystals and cause these slices to be unfit for grade one wafer product.
As such, a need exists in the semiconductor industry for a process of preparing a silicon melt for growing a single silicon crystal wherein the silicon melt contains a very low amount of gases insoluble in silicon.
Among the objects of the present invention, therefore, are the provision of a process for preparing a silicon melt containing a very low level of gases insoluble in silicon; the provision of a process for preparing a single silicon crystal containing a very low level of large crystal voids; the provision of a process for producing a silicon melt which produces a high percentage of grade one wafers; the provision of a simple, cost-effective process which reduces the number of defects in a grown single silicon crystal; and the provision of a process for preparing a silicon melt in which substantially all of the gas trapped in the silicon charge during the melting process is soluble in silicon.
The present invention, therefore, is directed to a process for controlling the amount of insoluble gas trapped by a silicon melt. The process comprises first charging a crucible with polycrystalline silicon and heating the crucible to melt the charge. During the melting of the polycrystalline charge a purge gas is flowed into the polycrystalline charge. The purge gas has a mole fraction of at least 0.1 of gas having a solubility in silicon of at least about 1xc3x971013 atoms/cm3.
The present invention is further directed to a process for controlling the amount of insoluble gas trapped by a silicon melt wherein a crucible is first charged with polycrystalline silicon and the crucible heated to melt the charge. A purge gas having a mole fraction of at least 0.1 of a gas having a solubility in silicon of at least about 1xc3x971013 atoms/cm3 is flowed into the charge during a heating phase and a melting phase of the polycrystalline melting process. The heating phase comprises the time period during the melting of the silicon before molten silicon is formed and the melting phase comprises the time period from the formation of molten silicon until the polycrystalline silicon charge is completely molten.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.